Frequency shift keying circuits



April 28, 1953 J. B. SINGEL HAL 2,636,941

- FREQUENCY SHIFT KEYING CIRCUITS Filed April 25. 1950 ATTORNEY PatentedApr. 28, 1953 FREQUENCY'v SHIFT KEYING. CIRCUITSy John Bx. Singel,Robert E. Leister,y and,v JamesR. Heck; Baltimore, Md., assignors toWestingf `house Electricl Corporation,y East` Pittsburgh', Pa.,. acorporation of Pennsylvania ApplicationiAprilzZS, 1950; SerialNo.157;918

.This` invention relates:l to keying, systems; for

radioy frequency; oscillatorsl and more: particularlyY to a. frequencyshift system forV keying ra:- dio frequency oscillators.

An objectI is to provide an improvedl frequency shift system. forAkeying radio frequency: oscil laters;

In thexpresent: invention frequency shift keying is accomplished. byshifting the; frequency offa crystal oscillator byawsmall amount. ThisisI accomplished by: connecting an extra capaci,- tance across the`crystal. is connected acrossthe; crystal by ar diodewhich is-inserieswith a keying'tube and akey. When thekeywis openy the keying tube isconductive and the diode also conducts current and provides av lowresistance connection between the crystal and theextra capacitance.Thiscau'ses the crystalV to 'oscillate at: a lower frequency. When thekey is closed the keying tube is connected to a bias obtained? from` theoutput off the oscillator- This biasrendersthe keying tubenon-conductiveand simultaneously applies a potential to the diodef insuch a direction as toincrease the for-V ward resistance of the diodetoa valuewhich will effectivelyv electrically disconnect the extracapacitance from the7 crystal. capacity across the crystal will slightlyraise the frequency of' the oscillator.

The oscillator systenrprovided'v by'this inven-L tion is required tooperate: fromaslow as'` 1'00- voltsl direct current.without` resortingto vibra-y tors 'rotating machinery; separateoscillator rectiiiers forbias potential`,`or losing the equiva lent in plate potential; thekeying circuit rev Itis a' still? further objectto provide a frequencylshift keying system-for a radio frequency'oscillator wherein keyingbiasis obtained fromthe output ofthe oscillator.

These and other objects: are eiected by our invention as will beizntfparent from thev follow ingI description 'amr claimsV taken inconnection`v`v with the accompanying drawing forming apart of this`application.

The figure; of" the` drawing is a schematic illustration of; a radiofrequency-'oscillator system employingk our invention.

The extra capacitancel This reduction in l'l of the oscillatorl tubey I.

Referring now to the drawing in detail, the;

reference' numeral represents an oscillator tube connected toazfrequency` determining cir-. The; oscillator tube has a cathode cuitlI3' l5,V acontrol grid l1, a screen grid i9. a-.sup-v pressor gridf 2|,and a plate 23. The frequency determining circuit I 3v comprises acrystal 25 shunted by serially connected capacitances 21' and 29. A 100`volt direct current. power supply 3| has its negative terminalconnected' to the cathode I5 ofthe oscillator tube through parallelconnected resistor 33and condenser 35. The network comprising theresistor 331 and the condenserprovides the proper bias for the grid TheYgrid l1 of the oscillator'tubeY l is: connected to the cathode l5through aresistor 31; The plate 23' ofthe oscillator tuloeY Il isconnected to the positive terminal' of' the power supply 3| through thetuned' circuit comprising inductanceA 39 and variable.-

capacitance 4|. The screen grid I9 ofthe oscillator tube` il. isconnected to thepositive terminalr ofil the power supply 3| throughdropping 'resistor 43.

Anrampliner 45' having a cathode 41, a control grid' 4a a screen grid5|, a suppressor grid 53 and' aplate 551 is connected to the output ofthe oscillator tube` ll. The output from the oscillator tuber isconnected to the control grid dit of the amplifier 45 through aeouplingcondenser 51. is connected tothe negative terminal 63' of rthepower supply 3| through the parallel connected resistorA 59,v andcondenser 6|. The grid. 49 is connected through resistance 65; to apotential The network comprising the resistor' 59 and condenserdividerfwlnch,v is explainedv hereinafter.

plate E55. The screen grid 5| of the amplifier 45- is connected tothepositive terminal 1| of the power supply 3| through a dropping resistorv13.

Av second amplier 15, similar to the amplier- 45, having a cathode 11,`a control grid 19, a screen grid 8|, a suppressor grid 83 and a plate isconnected to -the output of the amplifier to. The output from theamplifierl is coupled" to the control grid of the amplifier 15 througha' coupling condenser 81. The cathode 11 of the second amplifier 15 is'connected to the negativeA terminal t3 of the power supply through a'`net- The cathode 41 ofi the ampliner 45' The inductance 61 and tuningcon-r work comprising a parallel connected resistor 89 and condenser 9|.The control grid 19 of the second amplifier 15 is returned to negativeline 63 through resistor 93. The plate 85 of the second amplifier 15 isconnected to the positive terminal 1| of the power supply 3| through aload impedance network comprisingv` an inductance 35 tuned by-a variablecondenser 91. -The screen grid 8| of the second amplifier 15 isconnected to the positive terminal 1| of the power supply 3| through adropping resistor- 99.

The output from the amplifier 15 is coupled to following stages of thesystem through coupling for radiating energy generated by theoscillatonThe suppressor grid of eachof the ytubes is connected directly to itsrespective cathode. The

pass paths for radio frequency to the negative line A... vn., .Y

i An inductance coil ||5 is inductively coupled to the inductance coil95 ingthe output of thel amplifier and picks up some ofthe energy fromthe output of the amplifier 15. This inductance coil ||5 is connected inseries with a resistance ||1 which is connected to the negative line 63.The resistance ||1 is provided with la Variable tap ||9 for picking oifvoltage developed by the inductance coil ||5.

A keyer tube |2| having a plate |23, a cathode |25 and at least acontrol grid |21, has its cath--v ode :|25 connected to a capacitance|29 in the frequency determining network, which has one of,v its platesconnected to the conductor connecting the crystalv 25 and thecapacitance 2?, and has its other plate connected to one side of a diode|3|. The other side of the diode |3| is connected to condenser 21opposite'theplate connested to the junction of the crystal 25 and theconnections of the condenser |29. The plate |23 ofthe tube |2| isconnected to the positive terminal of the power supply 3| through aresist-- ance |33.

' The voltage vinduced in the inductance coil 5 is picked'oif theresistance ||1 and'rectied by av diode |35. This voltage appears acrossa potential divider comprising resistances |31 and |39. A condenser |4|is connected acrossfthe resistances 31 and |39 and provides a filter.vThe cathode |25 of the keyer tube |2| isA connected to the junctionpoint of the resistances |31 and |39 through a resistance |43. The grid|21 of the keyer tube |2| is connected tothe 'negative end of ,theresistance |31 through a single pole single` throw key |45. It'will alsobe observed"that the'grid |21 of the keyer tube is connected to theplate |23 of the keyer'tube"|2| at point A through a high resistance|41.

To'explain the operation of our oscillator system, assume that thecircuit is adjusted until the potential of point A is 5 volts positiveand key |45 is open. It is observed that plate |23, and control grid |21will both have this magnitude of potential applied. The resistance |41however, is a relatively high resistance. With these conditionsexisting, the keyerrtube |2| will be conductive and current will nowfrom point A through the keyer tube |2| from the plate |23 i to cathode25, through conductor |49 to the diode 53| in thel frequency determiningnetwork, to the negative terminal 63 of the power supply 3| through line|5|, through the power supply 3| to the positive line 1| and through theresistance 33 back to the point A. The current underthese conditions isapproximately 5 to 10 milliamperes. This current reduces the forwardresistance of diode |3| in the frequency determining network toapproximately 50 ohms. This low resistance is substantially equivalentto a short circuit. Under this condition, the diode |3|l conductscurrent and connects the capacitance |29 in parallel with'thecapacitance 21 and the frequency of the crystal is shifted. The additionof the capacitance |29 in parallel with the capacitance 21 and thecrystal 25 will slightly lower the frequency of oscillation of thecrystal 25.

The radio frequency potential induced in the inductance coil 4H5 andpicked off resistance ||1 is rectified by the diode |35 and appears as avoltage acrossthe potential divider comprising the resistors |31 and|39. The position of the variable contact ||9 on the resistance ||1controls the magnitude of. the voltage appearing across the resistors|31land |39. In a practical setv up,"thev'ariable contact'. i9. adjustedto give approximately l5 volts across resistance |31 and l5 volts acrossresistance |39 with the negative end being adjacent the key 145.

When the'key |55 is closed, a potential of 30 volts'minus is applied tothe control grid |21 to cut oi the plate current, which ceases to iiowthrough tube |2|. This cessation of platey cur- V rent flow causes thecathode |25 to change its ,v l5 volts minus which is still suiiicient tobias the 35A grid |21 of the keyer tube |2| to cut off and stop the flowof plate current. Y in addition tov *When 'the key |45 is closed,stopping the flow of plate current through the diode |3| in thefrequency determining network,H closing of the key also connects thecathode |25l of the keyer tube |2| to the junction of the resistors |31and |39 which point is minus-l5 volts below the negative terminal of thepower supply; 3|. This negative potential is applied'through-yresistance |43 to the diode |3| in the frequency determining network.Under these conditions,4

the voltage impressed across the diode |3| in the frequency determiningnetwork is now in the reverse direction from that impressed on thediode'v |3| with the key |4'5openf Consequently, with the key |45closed, the effective resistance of the diode |3| is several hundredthousand ohms. This high resistance is effectively an open circuit andAelectrically disconnects the capacitorg|291 from across the frequencydetermining network' WI-ithgthe capacitor |29 disconnected from acrossthe `frequency determining network, ythe fre- 'f quency determiningnetworkewill oscillate at a higher frequency than when the key |45 isopen.

It is seen that by means of a single contactthrough the contacts of thekey |45 may be as.

low as a small fraction of a milliampere.

From the foregoing description taken in connection with the drawing, itis seen that we have provided an improved frequency vshift keying .y

system for keying an oscillator which operates from a low potentialsource of direct current by obtaining bias for operating the keyer tubefrom a portion of the oscillator output.

While we have shown our invention in only one form, it will be obviousto those skilled in the art that it is not so limited, but issusceptible to advance changes and modifications without departing fromthe spirit and scope thereof, and We desire, therefore, that only suchlimitations shall be placed thereon as are specifically set forth in theappended claims. For example, by means of diode itl, capacitor l29 isconnected in parallel with 2l. By means of another capacitor equivalentto 429, another diode equivalent to 13| and another keyer tube andassociated circuits, this second capacitor could be electronicallyconnected in parallel with capacitor 29 to give a still greaterfrequency shift of the crystal.

We claim as our invention:

1. In a frequency shift oscillator system, a frequency determiningnetwork comprising a rst impedance, a second impedance adapted to beconnected in parallel with said rst impedance to shift the frequency ofsaid frequency determining network, a diode connected between said firstand second impedances, a keying tube connected to said diode in suchmanner that when said keying tube is made conductive said t.;

diode is also made conductive and connects said second impedance inparallel with said .first irnpedance, a potential divider connected tothe cathode of said keyer tube, means utilizing a portion of the outputfrom said frequency determining network for developing a potentialacross said potential divider, a key connected between said potentialdivider and the grid of said keyer tube which upon closing appliessimultaneously a potential to the grid 0f said keyer tube to render thesame non-conductive and a potential to said diode in a direction to makesaid diode effectively a high impedance which electrically disconnectssaid second impedance from said rst impedance.

2.1n a frequency shift oscillator system, a

frequency determining network comprising a permanently connected firstimpedance, a second impedance adapted to be connected in parallel withsaid first impedance to shift the frequency of oscillation of saidfrequency determining network, a diode connected between said rst andsecond impedances for connecting said second impedance in parallel withsaid rst impedance, a keying tube connected to said diode in such mannerthat when said keying tube is made conductive said diode is also madeconductive and connects said second impedance in parallel with saidfirst impedance, a potential divider, a rectiiler connected to theoutput of said frequency determining network for rectifying a portion ofthe output from said frequency determining network, means for impressingthe rectied output from said rectifier across said potential divider, akey connected to the negative Vend of said potential divider in suchmanner that upon closing of said key potential is applied simultaneouslyto the grid of said keying tube to render the same non-conductive and tosaid diode in such a direction as to effectively make said diode a highimpedance which isolates said second impedance from said rst impedance.

3. In a frequency shift oscillator system, a frequency determiningnetwork comprising a permanently connected first impedance, a secondimpedance adapted to be connected in parallel to shift the frequency ofoscillation of said frequency determining network, a diode connectedbetween said rst and second impedances, a keyer tube connected to saiddiode in such manner that when said keyer tube is conductive said diodeis also conductive and connects said second impedance in parallel withsaid first impedance, an amplifier connected to amplify the output fromsaid frequency determining network, a potential divider, meanscomprising a rectiner for applying a portion of the output from saidampliner across said potential divider, and a key connected to thenegative end of said potential divider and adapted when closed to applya negative blocking potential to the grid of said keyer tube to rendersaid tube non-conductive and simultaneously apply a negative potentialto said diode 4in said frequency determining network in such a directionas to effectively make said diode a high impedance which isolates saidnrst impedance from said second impedance.

4. A frequency shift keying system comprising an oscillator having afrequency determining network, said network including a reactiveimpedance element in series with a diode, means for applying potentialto said diode in one direction to cause it to present a low impedanceand in the opposite direction to cause it to present a high impedance,said means comprising a keyer tube having an anode, a cathode and acontrol grid, means rendering said tube normally conductive, meansconnecting said tube in series with said diode, a voltage divider, meansfor deriving a direct current bias potential from the output of saidoscillator and applying said potential to said divider, means connectinga key between said grid and the negative terminal of said divider, meansconnecting the intermediate terminal of said divider to said cathode andto one side of said diode, and means connecting the positive terminal ofsaid divider to the other side of said diode.

JOHN B. SINGEL. ROBERT E. LEISTER. JAMES R. HECK.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,426,295 Born Aug. 26, 1947 2,462,181 Grosselnger Feb. 27,1949 2,474,261 Leibe June 28, 1949 2,492,791 Finch Dec. 27, 1949

